1. Technical Field
The present invention relates to a brushless motor having no commutator for detecting a rotational position of the motor.
2. Related Art
A brushless motor is used that has a semiconductor switch instead of a brush serving as a commutator because of problems of maintenance and sparks. Particularly, a brushless motor is known that has no encoders for detecting a rotational position of the motor and sensors utilizing a hole effect.
FIG. 11 is a block diagram of a motor driving portion and a motor driving circuit of a related art motor as an example of such brushless motor.
As shown in FIG. 11, a brushless motor (dc motor without commutator) 1 includes a motor driving portion 2 and a driving circuit 3. The driving circuit 3 includes an induced voltage detection portion (back electromotive force detection unit) 4, a phase shift portion (pulse delay unit) 5, a driving pulse generating logic portion (logic pulse generating unit) 6, and a switching circuit portion (stator winding power supply unit) 7.
The induced voltage detection portion 4 detects an induced voltage generated in a non-energized interval of each of driving coils (stator windings) 2a of the motor driving portion 2, and outputs a timing of a zero-cross point of an induced voltage waveform in each phase as a pulse train. The phase shift portion 5 shifts a phase at a timing at which an output pulse train is applied to each driving coil 2a from the induced voltage detection portion 4. The driving pulse generating logic portion 6 generates a driving pulse for each phase, i.e., each coil 2a based on the output pulse train from the phase shift portion 5. The switching circuit portion 7 properly supplies a driving voltage to each driving coil 2a based on the driving pulse. Refer to JP-A-3-230791 (claims 3 and 4, and FIG. 1).
The brushless motor utilizing an induced voltage without using a sensor, however, has the following problems. It is difficult to use the brushless motor in a single-phase full-wave drive in which non-energized interval does not occur in a coil. The driving system is cumbersome, so that design costs increases because the brushless motor includes a unit for detecting a zero-cross point, an arithmetic portion of a phase shift, and the like. Additionally, a method detecting only a zero-cross point in a non-energized interval from an induced voltage has a problem in that an induced voltage waveform similar to a sine wave is not effectively used.